Method for measuring concentration of dopant within a semiconductor substrate

ABSTRACT

A method is provided for measuring at resolutions which are in some instances less than 20 nanometers the concentration densities within one or more diffusion regions within a semiconductor substrate. The diffusion regions are prepared for measurement by cleaving a cross-sectional surface and polishing that surface to a substantially flat, exposed profile. The profile is purposefully pre-etched to remove oxide abutting the implant area and thereafter dopant-selective etched in accordance with concentration densities within the substrate. Pre-etching of oxide and concentration density etching of doped silicon provides an exposed topological contour measurable by atomic force microscopy (AFM). AFM can detect the entire cross-sectional surface including conductors and dielectrics. The topological height of impurity region profiles of a calibration wafer are correlated to impurity concentrations to form a calibration curve. The calibration curve, in conjunction with topological contour of a target region profile, allows direct and quick measurement of concentration densities along the target region profile at each AFM scan location. The initial scan position is purposefully defined by an oxide pre-etch step to present an easily discernible AFM-read gradient which signals an initial AFM read position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuit manufacture and moreparticularly to a method for measuring dopant profile characteristicswithin a semiconductor substrate.

2. Description of the Relevant Art

Manufacture of an integrated circuit begins by diffusing or implantingwith impurity ions one or more isolated regions across a semiconductorsubstrate. When activated through temperature anneal, the implantedregions become conductive. Deposited on the upper surface of one or moreconductive regions is a plurality of interconnect conductors. Theimplanted regions and overlying interconnect form a monolithicintegrated circuit well known in the art.

Modern integrated circuits employ densely patterned electronic devicespatterned across the semiconductor substrate. Each device is quite smalland, in some instances, is less than only a few square microns in area.Smaller devices dictate small dopant regions closely spaced across amajority if not the entire wafer. The dopant regions can be formed inany type of semiconductor substrate including, for example, silicon andgallium arsenide. The dopant regions are formed using either n-type orp-type impurities such as, for example, boron, phosphorus, arsenic,antimony, boron fluoride, etc. The dopant regions are formed by activelyinjecting or passively diffusing a desired impurity into one surface ofthe substrate. Modern fabrication techniques generally use an ionimplanter for actively injecting impurities into the dopant regions.Medium to high energy implanters can easily implant the impuritiesthrough a thin oxide naturally formed or purposefully placed on thesubstrate upper surface. If the substrate is silicon, then a nativeoxide occurs when the substrate surface is exposed to oxygen. In manyinstances, the substrate surface is purposefully allowed to grow a thingate oxide in addition to or in lieu of native oxide.

The electronic devices formed on the substrate surface are generallyclassified as passive or active devices. An active device generallyincludes a transistor, whereas a passive device generally includes acapacitor or resistor. Whether active or passive, functionality thereofis determined by many factors. An important factor is the concentrationof dopant atoms existing within the dopant regions. To determineresistivity of the dopant regions, turn-on characteristics of atransistor channel between dopant regions, parasitic capacitance at thejunction between dopant regions and substrate, susceptibility of hotcarrier injection from dopant regions, as well as other characteristics,it is necessary to measure concentration of dopant/impurity atoms in thedopant regions. Periodic measurement of the dopant regions allowsengineers and scientists to predict operability of the ensuing device.

There are numerous ways in which to characterize the dopant region. Oneway is to profile a cross-section of the wafer severed along the dopantregion to expose that region extending from a point just below thesurface oxide a distance toward the wafer backside surface. The dopantregion profile (referenced hereinafter as "dopant region") therefore hasan exposed cross-section surface extending perpendicular (i.e.,vertically into the substrate from the oxide) and lateral (i.e.,horizontally) to the oxide. Measuring dopant concentration, therefore,begins by measuring concentration in two dimensions along both thevertical and lateral directions.

Determination of two dimensional impurity concentration along the dopantregion has evolved over the years. Early measurement techniquesgenerally measure resistance and convert each resistance reading to aconcentration amount. Resistance-to-concentration conversion requireswell known spreading resistance probe (SRP) measurement and conversiontechniques. SRP uses a pair of probes brought in contact with thesurface of the dopant region. A voltage of several millivolts is appliedacross the probes and a resulting current is measured. A resistivity ofthe contacted dopant region can then be measured and related toconcentration as a function of carrier mobility. While SRP techniquesprovide beneficial readings of dopant concentration levels at thesurface of the dopant region, SRP techniques suffer many disadvantages.Firstly, SRP probes can only measure in a single dimension betweenprobes the spreading resistance at the surface of the dopant profile.Secondly, and more importantly, failure of SRP probes to adequatelycontact the dopant region profile will deleteriously affect resistivereadings. Thirdly, SRP measurements are limited by the resolution of theprobe tip geometry and, since the probe tip diameter must be large tocarry adequate measuring current and voltage, SRP measurements aregenerally limited to resolutions exceeding several microns.

Another widely used dopant characterization technique is secondary ionmass spectrometry (SIMS). SIMS can directly measure, albeit at lowlateral resolutions, the dopant concentration in two dimensions not onlyat the surface region but as a function of depth. SIMS techniqueconsists of using an ion beam (usually oxygen or cesium) to sputter awaylayers of the dopant region. The sputtered dopant region produces ionsthat can then be mass analyzed. Sensitivity of the mass analyzer islimited by interference between desired ions and ion complexes. Thisproblem is particularly acute when measuring, for example, ³¹ P and SiHcomplexes having somewhat similar atomic mass. A more importantlimitation of SIMS measurement is lack of resolution. SIMS resolution islimited by the diameter of the sputter beam impinging the dopant region.Generally speaking, SIMS sputter beam diameter oftentimes exceedone-half micron (μm) thereby limiting SIMS usefulness to dopant regiongeometries which are on the order of one micron in lateral and verticaldimension. In order to determine a concentration gradient across a smalldopant profile less than, for example, one half micron, a higherresolution measurement technique is needed.

Responsive to the need for higher resolution, many researchers havediscovered the interrelation between dopant selective etching andtopography profiling. In an article to Takigami, et al., "Measurementsof the three-dimensional impurity profile in Si using chemical etchingand scanning tunneling microscopy", Appl. Phys. Lett. 58 (20), May 20,1991, researchers have pointed to the advantages of using an etchantwhose etch rate depends upon impurity concentration for removing aportion of the dopant region profile. Takigami, et al. describe theimportance of cleaving the dopant region to obtain a dopant regionprofile, etching the profile with a concentration-dependent etchant, andthen measuring the resulting upper topography with a scanning tunnelingmicroscope (STM). The etchant is capable of removing the doped substratewith high resolution demarcation, and the STM is capable of measuringthe resulting topography also at high resolution.

The principal of STM operation generally comprises an atomically sharptip brought near enough to the dopant region surface that the vacuumtunneling resistance between the surface and tip is finite andmeasurable. The tip scans the surface in two dimensions, similar to araster pattern, while the height is adjusted to maintain a constanttunneling resistance. In order to maintain the constant tunnelingresistance, the metal tip is displaced by a feedback voltage read fromthe tip to various piezo drives connected to move the tip. The piezodrives move the probe tip toward or away from the surface as it is beingscanned across the surface to yield a contour map of the surface.

STMs in combination with concentration-dependent etching provideadvantages of higher resolution profiling of the etched surface. Oncethe topography is known, it is then related to an impurity concentrationusing a known, unetched region under, for example, a protectivephotoresist layer. The unetched region being used as a calibrationsurface is generally located on the surface of the wafer and not on thescanned, profile area being measured. Comparison between dissimilarsurfaces prepared under dissimilar conditions is an inappropriate way inwhich to calibrate a test procedure. The problems of using a dissimilarsurface as the calibration surface are manyfold. Firstly, use of aphotoresist material to protect the calibration surface renders causticmaterial on the wafer, which in some instances cannot be removed at theto-be-measured, critical cross-section. Secondly, any photoresist lefton the calibration surface or the cross-section surface can skew thehigh resolution STM profile, thereby defeating the purpose of STM.

Related to the problems of preparing a surface in which to calibrateimpurity concentration to etch depth of the dopant region, STM simplycannot measure all types of dopant profiles. The dopant region (i.e.,dopant profile) generally includes various dielectric and conductivecross-section layers. Dielectric and conductive layers are formed duringwafer fabrication on one surface of the wafer on which a cross-sectionis thereafter exposed for concentration readings. STM, due to thenecessity for having two conductors between the tunneling path, can onlymeasure the topography of a conductive surface. The dielectric oxideadjacent the dopant profile cannot be detected by the STM probe. Sincetunneling current does not exist between the probe and the oxide, theprobe can inadvertently contact the dielectric and possibly break uponcontact. STM is therefore of limited use when measuring a dopant regionformed according to normal processing steps having dielectric layersexposed in the profile being measured.

In an effort to overcome limitations of STM, atomic force microscopy(AFM) was discovered capable of measuring topography of both conductorsand dielectrics. Raineri, et al., "Carrier distribution in silicondevices by atomic force microscopy on etched surfaces", App. Phys. Lett.64 (3), Jan. 17, 1994. Raineri, et al. describe AFM used to profile adopant region etched in accordance with dopant concentration. While AFMprofiling has numerous advantages, it is nonetheless limited by thecalibration technique used.

In essence, a benchmark etch depth must be correlated to an impurityconcentration. The benchmark etch depth and corresponding impurityconcentration must be formulated from a calibration surface. Given abenchmark etch depth and corresponding impurity concentration, any etchdepth reading in a to-be-measured (i.e., target) dopant region about thebenchmark can then be correlated to an impurity concentration value. Toobtain reliable target readings, it is therefore necessary to maintainthe integrity of the calibration surface. It is also necessary that thecalibration surface be prepared under similar processing constraints asthe target surface being measured. Thus, it would be desirable to form acalibration surface having a dopant concentration profile similar to thetarget concentration profile. If the target and calibration surfaces arenot flat and are not: prepared under similar processing constraints,then AFM readings on the target region will not calibrate back to anaccurate impurity concentration derived from the calibration surface.

Of further importance in calibration is the need for establishing aninitial scan reading location consistent on both the calibration andtarget surfaces. It is desirable that the scanning of both thecalibration and target surfaces be initialized at a known locationidentical with each other. Further, the reading location must be formedsuch that scanning of the AFM probe and readings therefrom are notcompromised by large disparities at the upper surface being scanned. Itis therefore essential that a reading location adjoining both thecalibration and target dopant profile be properly fashioned to minimizedisparity of the upper scanned surface and to maximize measurabilitywith the highest possible detection resolution.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by an AFMcalibration methodology of the present invention. That is, AFMcalibration hereof utilizes a calibration surface prepared in accordancewith similar processing constraints as the upper target surface to bemeasured. The calibration surface is formed from a cleaved calibrationwafer fabricated similar to a target wafer to be measured. The targetwafer is cleaved and prepared along the cleaved, cross-sectional surfaceidentical with the calibration cleaved surface to present a targetdopant region profile on the cross-sectional target wafer. Both thetarget and calibration dopant regions (i.e., profiles) are etched usinga concentration-dependent etchant, and the resulting topography on bothregions are measured using an AFM probe. The AFM probe is adapted formeasuring the amount of material removed from the calibration regionprofile at a specific location along that profile--i.e., relative to aknown initial read location. The amount of etch removal at each scanlocation on the calibration region can then be correlated to an impurityconcentration by using an SRP probe or SIMS upon a profile of a teststructure, wherein the test structure is formed under identical orsimilar process conditions as the calibration region. Correlation toimpurity concentration at that specific location provides a benchmarketch removal amount corresponding to a benchmark impurity concentration.Any change in etch amount about the benchmark etch amount can then becorrelated to variations in impurity concentration and presented as acalibration curve derived from the calibration region. The target regionprofile and AFM readings therefrom can then be correlated, using thecalibration curve, to impurity concentrations at any location along thetarget region--i.e., relative to a known initial read location.

AFM readings on the calibration and target surfaces are consistentlymaintained therebetween knowing a starting position for the AFM rasterscan. In essence, the AFM probe begins valued measurements when aninitial read location is detected. The read location of the calibrationdopant region and target dopant region are taken from respectiveprofiles at the junction between an oxide and the calibration and targetdopant regions. The oxide in both regions is formed using, for example,identical processing steps to ensure the initial read location isidentical for scanning of both regions. Once the initial read locationis determined, probe scan along the respective profile is always relatedback to the known read location so as to compare lateral scan distanceon the calibration dopant region to lateral scan distance on the targetdopant region and vice versa. Absent identical starting positions forthe calibration scan and target scan, the operator cannot relate etchdepth and impurity concentration at one lateral distance on thecalibration surface to etch depth and lateral distance on the targetsurface necessary to perform true calibration. Of importance to thepresent outcome, therefore, is not only the accurate calibration ofimpurity concentration to etch depth, but also accurate presentation ofa consistent starting scan location for both the calibration scan andthe target scan.

Broadly speaking, the present invention contemplates a method forderiving a concentration density of dopant within a dopant region as afunction of etching depth into the dopant region. The method comprisesthe steps of providing a wafer having an upper surface and an oxideformed thereon, and a dopant region extending from the upper surface toa depth below the upper surface. The wafer is then cleaved perpendicularto the upper surface to expose a cross-section of the oxide and thedopant region. The wafer is next exposed to an oxide etchant and then toa dopant region etchant to remove along the cross-section a portion ofthe oxide and the dopant region. The dopant region is removed at a rateproportional to a concentration of activated dopant atoms within thedopant region. As defined herein, "activated" dopant atoms are thoserendered electrically active by exposing the dopant to temperaturecycles associated with anneal. The oxide and the portion of the dopantregion abutting the oxide are removed to a topological height disparity,according to one exemplary embodiment, greater than 10 nm and less than40 nm. A probe of an atomic force microscope is then scanned across thecross-section. The topological height disparity signals to the atomicforce microscope the uppermost boundary of the dopant region and therebypresents an initial read location. Concentration of the dopant withinthe dopant region can then be computed from the read location signalingthe uppermost boundary to depths below the uppermost boundary as afunction of the amount of dopant region removed by the dopant regionetchant.

The present invention further contemplates a method for correlatingdopant concentration and etchant depth of a known (calibration) dopantregion profile to a to-be-determined (target) dopant region profile. Themethod includes the steps of preparing a cross-section of a targetsemiconductor substrate having a target region of dopant formed,according to one embodiment, simultaneous with and at the sameconcentration as the dopant placed within the calibration semiconductorsubstrate calibration region. An oxide arranged upon the cross-sectionof the target semiconductor substrate is removed to a reduced height andthereafter a portion of the target region is removed with an etchanthaving an etch rate dependent upon the concentration of the dopantwithin the target region. Partial removal of the target region and theoxide upon the target semiconductor substrate occur simultaneous withand under identical conditions and duration as each of the steps forpartially removing calibration region material and oxide abutting thecalibration region material formed on the calibration semiconductorsubstrate. The upper surface of the target region is then measured byscanning a probe from an initial position adjacent the reduced heightoxide (i.e., start location) of the target semiconductor substrate to anend location at least partially across the target region whileperiodically taking readings of the upper topography. A concentration ofdopant within the target region is then computed from the calibrationcurve by correlating an etch amount taken from each reading of the uppertopography of the target region with the corresponding read locations onthe upper topography of the calibration region.

The present invention still further contemplates a calibration wafer anda target wafer cleaved and polished along a surface which issubstantially flat, and is perpendicular to a backside surface of eachrespective wafer. After cleaving and polishing the calibration andtarget wafers, a calibration dopant region profile is presented from thecalibration wafer which, according to one embodiment, is larger incross-section than a target dopant region profile of the target wafer.The calibration and target region profiles can have n-type impuritiesor, conversely, p-type impurities implanted therein. Adjoining thetarget and calibration dopant region profiles are respective target andcalibration oxides. The target and calibration oxides are preferablysilicon dioxide, in substantially stoichiometric proportions.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIGS. 1A and 1B are top plan views of a calibration wafer and a targetwafer;

FIGS. 2A and 2B are cross-sectional views along planes 2A and 2B,respectively, of FIGS. 1A and 1B;

FIG. 3 is a flow diagram of the steps used for deriving a calibrationcurve from the calibration wafer and for determining from thatcalibration curve and AFM readings on the target wafer a target waferdopant concentration profile according to the method of the presentinvention;

FIG. 4 is a perspective view of target or calibration region profilesundergoing topological AFM scanning according to the present invention;

FIG. 5 is a graph showing topological height versus depth of the targetor calibration region profiles before and after dielectric pre-etch anddopant-selective etch according to the present invention; and

FIG. 6 is a calibration curve derived from the calibration waferaccording to the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIGS. 1A and 1B, top plan views of calibration wafer 10and target wafer 12 are shown. Calibration wafer 10 is preferably formedaccording to the same or similar processing specifications, steps,diffusion densities, temperature cycles, etc. as target wafer 12. In oneembodiment, calibration wafer 10 and target wafer r12 can be taken froma dissimilar process run or batch. According to another embodiment,calibration wafer 10 and target wafer 12 are each fabricated in the sameprocess run or batch in order to take on substantially identical deviceoperation characteristics. In yet another alternative embodiment,instead of having two wafers on two dissimilar monolithic substrates,the calibration substrate can be identical with the target substrate,wherein both calibration and target cross-sections are taken from thesame wafer. In the latter situation, identical process parameters areguaranteed. In either instance, however, it is merely necessary topresent a calibration cross-section similar to a target cross-section,and that each section be substantially flat to allow topologicalreadings therefrom.

If, according to the preferred embodiment, a separate wafer is used asthe calibration wafer, then the calibration wafer is cleaved along plane14. Similarly, target wafer 12 is cleaved along plane 16. Plane 14 aswell as plane 16 is chosen perpendicular to the backside surface ofrespective wafers 10 and 12. Plane 14 and 16 are cleaved using a scribeoperation. A diamond-bonded wheel provides a suitable work surface.Additional polishing, if desired, can be done using a frosted glasswheel with water, a mechanical wheel in conjunction with a fluid slurry,or a combination of chemical etching and mechanical abrasion. A popularsilicon chemical-mechanical polishing technique employs fine silicapowder in a hydroxide solution. An analogous process for galliumarsenide is silica in a sodium hypochlorite solution. Either instancewill provide a substantially flat cross-sectional surface, a portion ofwhich is shown in FIGS. 2A and 2B.

Referring now to FIGS. 2A and 2B, a portion of the cross-sectionalsurfaces taken from wafers 10 and 12 of FIGS. 1A and 1B, respectively,are shown. FIGS. 2A and 2B illustrate cross-sectional or "profile"surfaces 18 and 20, respectively. Cross-sectional surface 18 is denotedas the profile of calibration wafer 10, and cross-sectional surface 20is denoted as the profile of target wafer 12. Surface 18 therebyincludes cross-sectional surfaces of a calibration semiconductorsubstrate 18a and a calibration region 18b. Calibration region 18b isdiffused or implanted into the upper surface of calibration substrate18a according to common techniques. Calibration region 18b can beimplanted, and generally is implanted, through a thin dielectric 18cplaced on the upper surface of calibration substrate 18a. Dielectric 18cis preferably an oxide grown to a depth generally less than 500Angstroms and is suitable as a gate oxide. Placed on the upper surfaceof calibration oxide 18c are various levels of metallization 18d.Calibration metallization can extend as vias through oxide 18c viawindows or contacts directly to underlying calibration region 18b.

Similar to cross-sectional surface 18, cross-sectional surface 20 is aprofile surface along plane 2B of FIG. 1B. Surface 20 includes surfacesformed from target semiconductor substrate 20a, target region 20b andoxide 20c. Various layers and/or levels of metallization as well asdielectrics are placed over target oxide 20c, as shown by referencenumeral 20d. Various levels of metallization, with thick dielectricplaced therebetween, and contact of metallization conductors tounderlying calibration or target regions, 18b or 20b, respectively,complete the fabrication methodology involved in making an integratedcircuit. A plurality of integrated circuits are formed across a wafer.In one embodiment, calibration and target regions 18b and 20b,respectively, are taken from a cross-section of a single wafer or, in analternative embodiment, are taken from two separate wafers manufacturedaccording to identical or somewhat similar process parameters. Accordingto whichever embodiment is chosen, it is important that cross-sectionalsurfaces or profiles of at least one calibration region 18b and at leastone target region 20b are formed.

The need for a profile of respective calibration and target regions 18band 20b is evident by the methodology in which atomic force microscopy(AFM) is achieved. Topological readings are taken from the profiles ofcalibration and target regions 18b and 20b. Topological readings areachieved by scanning in a defined region 22 along a raster pattern shownby reference numeral 24. Scanning an AFM probe along pattern 24 allowsdetection of topological surface variation along cross-sectionalsurfaces 18 and 20 in defined regions 22. Regions 22 can be of almostany size, limited only by the time duration necessary to achieve AFM.Regions 22 are generally large enough to encompass at least a portion ofregions 18b and 20b, respectively. Topological scanning by AFM isfurther described in reference to FIG. 4. The necessity for AFM scanningand the preparation of calibration and target profiles of surfaces 18and 20 are described in reference to the steps set forth in FIG. 3.

Referring now to FIG. 3, a flow diagram of steps used for deriving acalibration curve from calibration wafer 10 and for determiningtwo-dimensional dopant concentration profile of target region 20b areshown. The methodology begins by completing the fabrication ofcalibration and target wafers using similar or identical processparameters, according to one embodiment, is set forth in step 26. Thecalibration and target wafers can then be cleaved at or near the pointof interest at step 28. Ideally, calibration and target wafers arecleaved along a cross-section which exposes profiles of calibrationregion 18b and target regions 20b, respectively. If cleavage does notallow such exposure, then grinding and/or polishing the cross-sectioncan be added to present such a profile. Thus, polishing step 30 may benecessary after cleaving step 28. Even if cleaving step 28 presents anadequate cross-section, polishing step 30 may nonetheless be needed topresent a perpendicular surface relative to the wafer backside surface.The perpendicular surface is substantially flat--a important outcome forAFM readings.

Once a flat cross-sectional surface is prepared at step 30, the profilesurface must then be etched using an etchant sensitive to oxide. Theoxide etchant, a suitable etchant being HF solution, is exposed as a wetetchant to the exposed wafer to etch back calibration oxide 18c as wellas target oxide 20c. Oxides 18c and 20c are thereby slightly recessedinto the cross-sectional surfaces 18 and 20. Pre-etching of the oxide,and the necessity thereof, is fully explained below in reference to FIG.5. Oxide etch back is needed to ensure the presence of an initialstarting position signaling the beginning of calibration and targetregion scan readings. The exposed oxide surfaces etched back to adesired depth is necessary to initiate a starting signal on the AFMdetector. The exposed oxide surface also reduces AFM tip artifactsallowing tip access to the highly doped region under the oxide. Anysuitable etch back amount which achieves that outcome falls within thespirit and scope of the present invention. Subsequent to pre-etch step32, a dopant concentration selective etch is performed at step 34. Asuitable wet etch solution includes 1 ml HF, 40 ml HNO₃ and 20 ml CH₃COOH. The etch rate of the etchant used in step 34 is dependent upon theimpurity concentration of the material being etched. Step 34 etchantthereby attacks and simultaneously removes exposed cross-sectionalsurfaces 18b and 20b. The etch rate is higher for high concentrationregions and lower for low concentration regions. Thus, after step 34, atopological gradient of dissimilar upper surface heights are therebypresented for AFM readings. The differential or changes in heightgradient indicates an etch differential and thereby a concentrationgradient.

Measuring the resulting topological features subsequent to step 34 isperformed using AFM techniques on both calibration wafer and targetwafer at steps 36 and 38, respectively. Steps 36 and 38 provide acontour of the topological features, wherein the calibration topologicalfeatures are read at each scan position and stored in memory. Each teststructure associated with calibration region 18b is thereafter probedusing SRP techniques to derive a resulting impurity concentration level.The calibration region positions can also be correlated to impurityconcentration levels using SIMS techniques. Either SRP or SIMStechniques provide correlation of etch depth at each position of thecorresponding test structure to an SRP or SIMS impurity reading. Etchdepth readings at each position along the calibration region teststructure are SRP or SIMS correlated in step 40 to benchmark impurityconcentration readings.

Once the benchmark etch depth and corresponding benchmark impurityconcentration is determined from the calibration wafer, a calibrationcurve is derived for the calibration wafer at step 42. The calibrationcurve represents etch depth versus concentration densities at variousscan positions across calibration region 18b, relative to an initialstarting position. The initial starting position is represented as aheight differential at the juncture between the calibration/target oxide18c/20c and the calibration/target region 18b/20b. The topologicalheight gradient at the oxide/dopant region juncture is more variable asa result of the pre-etch step 32 in relation to the impurityconcentration sensitive etch step 34.

An important advantage in deriving the calibration curve is the use of acalibration profile surface prepared substantially similar to oridentical with the target profile surface to be measured. The targetprofile is the actual region in which concentration readings are to betaken. The target region 20b can be made much smaller than thecalibration region 18b and, in many instances, is smaller than thecalibration region. Regardless of the size of calibration region 18bwith respect to target region 20b, both calibration and target regionsare defined as the profile regions of the respective wafers, and theprofile are those profiles cross-sectioned through substrates embodyingdopant concentrations. Dopant concentrations include any diffused orimplanted region of impurity atoms within a silicon or gallium arsenidesubstrate.

From the calibration curve obtained from calibration wafer at step 42,and given the AFM readings on target wafer at step 38, the AFM depthreadings can be automatically and quickly correlated to impurityconcentrations at each AFM-read location on the target wafer, as shownby step 44. Any etch-resultant contour point on target region 20b cantherefore be assigned a concentration magnitude by extrapolating etchdepth on the target wafer to a corresponding impurity concentrationamount taken from the calibration curve.

Details of AFM measurement is best described in reference to FIG. 4.FIG. 4 shows an AFM probe 46 movable in the X, Y and Z dimensions aboutcross-sectional surfaces 18 or 20. Surfaces 18/20 are oriented upward,as opposed to their horizontal orientation in FIGS. 2A and 2B. Surfaces18/20 are shown after oxide pre-etch step 32 and concentration-sensitiveetch step 34 of FIG. 3. Upper surface 18/20 thereby takes on an uppersurface contour proportional to the impurity concentration levels withincalibration/target regions 18b/20b. Oxides 18c/20c are also pre-etchedto minimize the gradient differential between the upper oxide topographyand abutting regions 18b/20b as well to reduce AFM tip artifacts.Fluctuations in height (H) are relative to changes in depth (D). Probe46 extends in a scan position numerous times across surfaces 18/20 asshown by arrows 48. Arrows 48 indicate a singular scan of raster pattern24 shown in FIGS. 2A and 2B. AFM readings are taken by raster scanningthe two-dimensional upper surfaces 18/20 in the X and Y dimensions.Repulsive or attractive forces between probe 36 and surfaces 18/20 aredetected by, for example, deflection of a cantilever (not shown)attached to probe 46 platform. Deflections of the cantilever are sensedusing suitable tunneling sensors or optical sensors. The repulsive orattractive forces generally arrive from an AC voltage at probe tip 46,from an externally applied magnetic field, or from adatoms havingmagnetic moment. A detailed description of atomic force microscopy (AFM)is incorporated herein by reference to Binnig, et al., "Atomic ForceMicroscope", Phy. Review Lett. Vol. 56, No. 9, Mar. 3, 1986. Movement ofthe cantilever provides resulting movement on probe tip 46 in theZ-dimension controlled by control unit 50. Control unit 50 provideselectrical stimulus to a piezoelectric drive unit 52 attached toplatform 54 of probe 46.

Turning now to FIG. 5 in conjunction with FIG. 4, a graph of topologicalheight (H) versus depth (D) of an actual surface is shown, such assurfaces 18/20 before and after dopant selective etch. Curve 56represents a profile of the height of surfaces 18/20 before etch steps32 and 34 of FIG. 3. As noted, curve 56 is relatively flat acrossregions 18b/20b. After etch steps 32 and 34 are completed, curve 58 isshown. Pre-etch step 32 effects curve 58 by reducing oxide surfaces18c/20c from curve 60 (shown in dotted line) to curve 58. Reduction inoxide height at pre-etch step 32 provides a noticeable yet lesseneddisparity DS₂ at the oxide/silicon juncture. Topological disparity atthe juncture is preferably greater than 10 nm and less than 40 nm.Absent pre-etch step 32, disparity DS₁ can be quite large, and isusually greater than 40 nm. Height disparity cannot always be determinedby AFM readings since the AFM scan cannot change instantaneously duringscan operation. Instead, during large disparity scans, AFM probe 46might not track the large etch-back region and may, in many instances,track the dashed line shown by reference numeral 62. Tracking along line62 defeats the purpose of AFM topological detection and concentrationsensitive etching. Since concentration sensitive etching is most severeat the surface region or juncture abutting the oxide, large disparitiesmight exist absent pre-etch step 32. Pre-etch step 32 and the adjustmentof oxide topography toward abutting silicon topography allows a smallerdisparity at the oxide upper surface and at the beginning ofcalibration/target regions 18b/20b. In addition, pre-etch allows tipaccess to the highly doped region immediately below the oxide.

FIG. 4 in conjunction with FIG. 5, shows maximum concentration at depthA with monotomicaly decreasing concentration to point B. Scan positionssubsequent to point B are relatively flat indicating low dopingconcentration. The curves 56 and 58 as well as the upper surfaces 18/20of FIGS. 4 and 5 are taken from AFM readings of calibration or targetregions, depending upon the surface being analyzed. FIG. 5 alsoillustrates curve 61 of concentration as a function of depth. Theconcentration is maximum at point A and decreases to point B.Concentration curve 62 is derived by SRP readings taken along the teststructure corresponding to upper surfaces 18/20 following, for example,the scan pattern of the AFM apparatus. In lieu of SRP, SIMS can be usedto determine concentration along the upper topography. Either techniqueallows concentration to be plotted as a function of depth. Knowingconcentration as a function of depth and height as a function of depthfor the scanned upper surface, a calibration profile can be determined.The scanned upper surface and readings therefrom are thereby taken froma calibration surface 18 of a calibration wafer 10. The calibrationsurface can therefore be analyzed at each height position to provide anetch amount versus concentration density.

FIG. 6 is a plot of concentration density taken as a function of etchamount. Etch amount corresponds to the height differential of plots 56and 58, and concentration density is taken at each height point asplotted on curve 61. A resulting calibration curve 64 is presented.Curve 64 can therefore be taken from a calibration region 18b beginningat an initial read location signaled by gradient disparity DS₂. As shownin reference to FIG. 3, calibration curve 64 provides a benchmarkconcentration versus etch amount for comparison of subsequent readingson the target region where the target region etch amount can then becorrelated to a concentration amount at each scan location relative tothe initial disparity gradient AFM signal. Accordingly, the method andprocedure hereof is well suited for automatically and quickly derivingan impurity concentration at any location across the target region givena pre-defined calibration curve taken from a similarly processedcalibration region.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that this invention is believed to be capable ofapplications with any type of semiconductor substrate having impurityregions implanted or diffused into one surface of the substrate. Theimpurity regions can be n-type or p-type, and the substrate can beeither silicon or gallium arsenide. It is also to be understood that theform of the invention shown and described is to be taken as exemplary,presently preferred embodiments. Calibration region can be taken fromthe same or from a dissimilar wafer as that used for forming the targetregion. Thus, the calibration region and target region can be taken fromcleaved surfaces of the same monolithic substrate or from dissimilarmonolithic substrates preferably fabricated in the same processing batchor run. Various modifications and changes may be made without departingfrom the spirit and scope of the invention as set forth in the claims.It is intended that the following claims be interpreted to embrace allsuch modifications and changes.

What is claimed is:
 1. A method for determining concentration of adopant within a target semiconductor substrate, comprising:preparing across-section of a calibration semiconductor substrate having acalibration region of dopant placed therein; etching to a reduced heightan oxide arranged upon the cross-section of said calibrationsemiconductor substrate; etching said calibration region with an etchanthaving an etch rate dependent upon a concentration of dopant atomswithin said calibration region; measuring the upper surface topographyof said calibration region by scanning a probe from an initial positionadjacent said reduced height oxide to an ending position at leastpartially across said calibration region while periodically takingreadings of the upper topography of said calibration region; andderiving a calibration curve of a concentration of dopant within saidcalibration region as a function of an etch depth determined from saidimmediately preceding step.
 2. The method as recited in claim 1, furthercomprising:preparing a cross-section of a target semiconductor substratehaving a target region of dopant formed at the same concentration as thedopant placed within said calibration region; and removing to a reducedheight an oxide arranged upon the cross-section of said targetsemiconductor substrate and thereafter removing a portion of the targetregion with an etchant having an etch rate dependent upon theconcentration of dopant within said target region, wherein partialremoval of the target region and the oxide upon the target semiconductorsubstrate occur simultaneous with and under identical conditions andduration as each of said steps for etching the calibration region andoxide upon the calibration semiconductor substrate.
 3. The method asrecited in claim 1, further comprising:preparing a cross-section of atarget semiconductor substrate having a target region of dopant formedwith processing parameters similar to those used to form the dopantplaced within said calibration region; removing to a reduced height anoxide arranged upon the cross-section of said target semiconductorsubstrate and thereafter removing a portion of the target region with anetchant having an etch rate dependent upon the concentration of saiddopant within said target region, wherein partial removal of the targetregion and the oxide upon the target semiconductor substrate occursimultaneous with and under identical conditions and duration as each ofsaid steps for etching the calibration region and oxide upon thecalibration semiconductor substrate; measuring the upper surface of thetarget region by scanning a probe from an initial position adjacent thereduced height oxide of said target semiconductor substrate to an endingposition at least partially across the target region while periodicallytaking readings of the upper topography of the target region resultingfrom said immediately preceding removing step; and computing from saidcalibration curve a concentration of dopant within said target region bycorrelating an etch amount taken from each reading of the uppertopography of the target region with corresponding readings taken fromsaid calibration curve.
 4. The method as recited in claim 1, whereinsaid preparing step comprises cleaving a calibration wafer to produce anexposed cleaved surface and polishing said exposed cleaved surface toform a substantially flat cross-section of said calibrationsemiconductor substrate.
 5. The method as recited in claim 4, whereinsaid polishing step comprises placing said exposed cleaved surface on apolishing surface arranged perpendicular to a backside surface of saidcalibration wafer.
 6. The method as recited in claim 3, wherein saidpreparing step comprises cleaving a target wafer to produce an exposedcleaved surface and polishing said exposed cleaved surface to form asubstantially flat cross-section of said calibration semiconductorsubstrate.
 7. The method as recited in claim 6, wherein said polishingstep comprises placing said exposed cleaved surface on a polishingsurface arranged perpendicular to a backside surface of said calibrationwafer.
 8. The method as recited in claim 3, wherein the steps of etchingand removing said oxide comprises exposing respective said calibrationsemiconductor substrate and said target semiconductor substratesimultaneously to a wet etchant sensitive to removal of only SiO_(x)(1≦x≧2) for removing oxide by an identical amount on said calibrationand target semiconductor substrates.
 9. The method as recited in claim3, wherein said measuring steps comprise scanning from a reduced heightoxide of respective said calibration and target semiconductor substratesto respective abutting said calibration and target regions.
 10. Themethod as recited in claim 1, wherein said deriving step comprisingdetermining concentration of dopant along said test structure of saidcalibration region at a plurality of locations along said upper surfacetopography.
 11. The method as recited in claim 10, wherein saiddetermining step comprises taking resistive readings along said teststructure and relating said resistive readings to concentration ofdopant.
 12. The method as recited in claim 10, wherein said determiningstep comprises sputter removing atoms of dopant from said test structureand mass analyzing the quantity of said sputter removed atoms.
 13. Amethod for deriving a concentration density of dopant within a dopantregion as a function of etching depth into said dopant region,comprising the steps of:providing a wafer having an upper surface withan oxide formed thereon and a dopant region extending from said uppersurface to a depth below said upper surface; cleaving said waferperpendicular to said upper surface to expose a cross-section to saidoxide and said dopant region; exposing said wafer to an oxide etchantand then to a dopant region etchant to remove along said cross-section aportion of the oxide and the dopant region, wherein said dopant regionis removed at a rate proportional to a concentration of dopant atomswithin said dopant region, and said oxide and a portion of the dopantregion abutting said oxide are removed to a topological height disparitygreater than 10 nm and less than 40 nm; scanning a probe of an atomicforce microscope across said cross-section, wherein said topologicalheight disparity signals to said atomic force microscope the uppermostboundary of said dopant region; and computing concentration of dopantwithin said dopant region from said uppermost boundary to depths belowsaid uppermost boundary as a function of the amount of dopant regionremoved by the exposing step.
 14. The method as recited in claim 13,wherein said computing step comprises determining resistivity of saiddopant region remaining after said exposing step and relating saidresistive readings to concentration of dopant therein.
 15. The method asrecited in claim 13, wherein said computing step comprises sputterremoving atoms of dopant from said dopant region remaining after saidexposing step and analyzing the quantity of said sputter removed atoms.16. A method for deriving a calibration curve of concentration densityversus etch depth on a calibration wafer and for relating etch depth ona target wafer to target wafer concentration density:formingcross-sections through calibration and target wafers having respectivecalibration and target oxides formed on one surface of said wafers andrespective calibration and target doping regions formed directly beneathsaid calibration and target oxides; exposing the cross-sections of saidcalibration and target wafers to identical etchant conditions to removea portion of the calibration and target oxides separate from the removalof a portion of the calibration and target regions, wherein a measurableupper surface disparity exists between said calibration oxide and saidcalibration region and well as between said target oxide and said targetregion, and wherein the calibration and target regions are removed at arate proportional to a concentration density of dopants therein;scanning a probe of an atomic force microscope across the cross-sectionsof said calibration and target wafers to determine respective etchdepth; deriving a calibration curve of concentration density versus etchdepth across the cross-section of said calibration wafer; and from etchdepth of said target wafer, relating comparable etch depth on saidcalibration curve to concentration density of said target wafer.
 17. Themethod as recited in claim 16, wherein said calibration region is largerin cross-section than said target region.
 18. The method as recited inclaim 16, wherein said calibration and target regions have n-typeimpurities therein.
 19. The method as recited in claim 16, wherein saidcalibration and target regions having p-type impurities therein.
 20. Themethod as recited in claim 16, wherein said measurable upper surfacedisparity is greater than 10 nm and less than 40 nm.